Sheet steel for use as packaging steel and method for producing packaging steel

ABSTRACT

The invention relates to sheet steel for use as packaging steel, made of a non-alloy or low-alloy and cold-rolled steel having a carbon content of less than 0.1%. According to the invention, in order to use such sheet steel for packaging steel that has good formability and can be produced in a cost-effective way, the sheet steel contains less than 0.4 wt % of manganese, less than 0.04 wt % of silicium, less than 0.1 wt % of aluminum, and less than 0.1 wt % of chromium and is provided with a multi-phase structure, comprising ferrite and at least one of the structure constituents martensite, bainite, and/or residual austenite. The invention further relates to a method for producing such packaging steel from cold-rolled sheet steel.

The invention relates to a sheet steel for use as packaging steelaccording to the generic part of Claim 1 and a method for producing apackaging steel from a cold-rolled sheet steel according to the genericpart of Claim 2.

Increasingly higher demands are made on the properties of metalmaterials for making packaging, in particular with regard to theirformability and strength. Indeed, so-called dual-phase steels are knownfrom automobile manufacturing; they have a multiphase structure thatsubstantially consists of martensite and ferrite or bainite, and they onthe one hand have high tensile strength and on the other also have highelongation at break. One such dual-phase steel with a yield point of atleast 580 MPa and an elongation at break A₈₀ of at least 10% is known,for example, from WO 2009/021898 A1. Because of the combination of thematerial properties of such dual-phase steels and high strength and goodformability the said dual-phase steels are especially suitable formaking complexly shaped and highly stressed components such as areneeded, for example, in the area of automobile chassis construction.

As a rule, the alloying of the known dual-phase steels is composed of amartensite fraction of 20% to 70% and any residual austenite fractionand also ferrite and/or bainite. The good formability of dual-phasesteels is ensured through a relatively soft ferrite phase and the highstrength is produced by the solid martensite and bainite phases, whichare bound into a ferrite matrix. In the case of dual-phase steels thedesired properties with regard to formability and strength can becontrolled in wide ranges through the alloy composition. For example, byadding silicon the strength can be increased by hardening the ferrite orbainite. Martensite formation can be positively affected through theaddition of manganese and the development of perlite can be prevented.The strength can be increased by alloying with aluminum, titanium, andboron. Moreover, alloying with aluminum is used for deoxidation and forbinding nitrogen that may be present in the steel. To develop themultiphase alloy structure dual-phase steels are subjected to arecrystallizing (or austenitizing) heat treatment, in which the steelstrip is heated to sufficient temperatures and then cooled so that thedesired multiphase alloyed structure is established with a substantiallyferritic-martensitic structural development. For economic reasons coldrolled steel strips are usually recrystallization annealed in acontinuous annealing process in an annealing furnace, where theparameters of the annealing furnace, for example pass-through rate,annealing temperature, and cooling rate, are established incorrespondence with the required structure and the desired materialproperties.

A higher strength dual-phase steel and a method for its production areknown from DE 10 2006 054 300 A1, where in the production process acold- or hot-rolled steel strip is subjected to a continuousrecrystallization annealing in a continuous annealing furnace in atemperature range of 820° C. to 1000° C. and the annealed steel strip isthen cooled from the said annealing temperature at a cooling ratebetween 15 and 30° C. per second.

Dual-phase steels known from automobile manufacturing are, as a rule,not suitable for use as packaging steel, since they are very expensive,particularly because of the high fractions of alloying elements likemanganese, silicon, chromium, and aluminum and since, for example, someof the known alloying elements must not be used for packaging steel inthe food area, since contamination of foods by diffusion of alloycomponents into the packaging contents must be excluded. In addition,many of the known dual-phase steels have strengths so high that theycannot be cold-rolled with the systems that are usually used forproduction of packaging steel.

Proceeding from this, the invention is based on the task of makingavailable a higher strength steel with good formability for use aspackaging steel that is as cost effective as possible to produce.Further, the invention is intended to point out a method for productionof a packaging steel that can be made cheaply and that has high strengthand high elongation at break.

These tasks are solved with a sheet steel having the features of Claim 1and with a method having the features of Claim 2. Preferred embodimentexamples of the sheet steel and the method for making it are indicatedin the dependent claims.

The sheet steel according to the invention for use as packaging steel ismade from a low-alloy and cold-rolled steel with a carbon content ofless than 0.1%. When used below, the term “sheet steel” will beunderstood to mean such a steel. Besides the low carbon content and lowconcentrations of the other alloy components, the sheet steel accordingto the invention is characterized by a multiphase structure, whichcomprises ferrite and at least one of the structural componentsmartensite or bainite. The steel from which the sheet steel according tothe invention is made can be a cold-rolled unalloyed or low alloy steel.Steels in which no alloy element exceeds an average content of 5% arecalled low alloy steels. The steel used to make the sheet steelaccording to the invention in particular has less than 0.5 wt % andpreferably less than 0.4 wt % manganese, less than 0.04 wt % silicon,less than 0.1 wt % aluminum, and less than 0.1 wt % chromium. The steelcan contain alloying additions of boron and/or niobium and/or titaniumin order to increase the strength, where the alloying with boronexpediently lies in the range of 0.001-0.005 wt % and the alloying withniobium or titanium lies in the range of 0.005-0.05 wt %. However,weight fractions that are <0.03% are preferred for Nb.

To develop the multiphase alloy structure the steel for making the sheetsteel according to the invention for use as packaging, steel is firstsubjected to recrystallization annealing by electromagnetic induction ata heating rate of more than 75 K/s and cooled after the recrystallizinginduction annealing at a cooling rate of at least 100 K/s. Through therecrystallizing heat treatment (with T_(max)>Ac1, since austenitizationis necessary) and subsequent rapid cooling there forms the multiphasestructure, which comprises ferrite and at least one of the structuralcomponents martensite, bainite, and/or residual austenite. The sheetsteel treated in this way has a tensile strength of at least 500 MPa andan elongation at break of more than 6%.

The recrystallizing (or austenitizing) annealing of the sheet steel bymeans of electromagnetic conduction proved to be an especially importantparameter for the production of the packaging steel according to theinvention. It was surprisingly established that the alloying of alloycomponents that are typically contained in dual-phase steels, forexample the alloying of manganese (which typically has a weight fractionof 0.8-2.0% in the known dual-phase steels), silicon (which typicallyhas a weight fraction of 0.1-0.5% in the known dual-phase steels), andaluminum (which is alloyed in a weight fraction up to 0.2% in the knowndual-phase steels) can be omitted if a cold rolled sheet steel withcarbon content less than 0.1 wt % is first subjected torecrystallization (or austenitizing) annealing at a heating rate of morethan 75 K/s by means of electromagnetic induction and then quenched at ahigh cooling rate of at least 100 K/s.

The surprising effect of the inductive heating on the development andarrangement of the martensite phase in the induction annealed steelstrip may be explained as follows: ferromagnetic substances are notmagnetized in the absence of an external magnetic field. However, withinthese substances there are regions (Weiss regions), which are magnetizedto saturation even in the absence of external magnetic fields. The Weissregions are separated by Bloch walls. Through the application of anexternal magnetic field first the favorably oriented, thus energeticallypreferred, Weiss regions grow at the expense of adjacent regions. TheBloch walls shift as this occurs. The electronic spin flip in this casedoes not take place everywhere simultaneously, rather the spins changedirection at the boundaries of the Weiss regions first. With a furtherincrease of the field, the direction of magnetization rotates into thatof the field until the spins correspond in all of the regions with thatof the external magnetic field and saturation is reached. It is alsoknown that a magnetic field can affect the motion of dislocations in theabsence of externally applied mechanical stresses. It now seemsplausible that the Bloch walls entrain carbon atoms and/or dislocationswhen they shift. Through this carbon and/or dislocations collect incertain areas, in which martensite forms after annealing and quenching.

Expediently, the sheet steel according to the invention for use aspackaging steel is fine or ultrafine sheet that was rolled to its endthickness in a cold rolling process. Fine sheet is understood to mean asheet with a thickness of less than 3 mm and an ultrafine sheet has athickness of less than 0.5 mm. After the recrystallization annealing andcooling, the sheet steel can be provided with a metal surface coating,for example of tin, chromium, aluminum, zinc, or zinc/nickel, toincrease its corrosion resistance. The known electrolytic coatingprocesses, for example, are suggested for this.

The invention is explained in more detail below by means of anembodiment example:

To produce embodiment examples of the sheet steel according to theinvention, steel strips of steels having the following composition [weremade], which were made in a continuous casting process and hot rolledand wound into coils:

-   C: max. 0.1%;-   N: max. 0.02%;-   Mn: max. 0.5%, preferably less than 0.4%;-   Si: max. 0.04%, preferably less than 0.02%;-   Al: max. 0.1%, preferably less than 0.05%;-   Cr: max. 0.1%, preferably less than 0.05%;-   P: max. 0.03%;-   Cu: max. 0.1%;-   Ni: max. 0.1%;-   Sn: max. 0.04%;-   Mo: max. 0.04%;-   V: max. 0.04%;-   Ti: max. 0.05%, preferably less than 0.02%;-   Nb: max. 0.05%, preferably less than 0.02%;-   B: max. 0.005%-   and other alloying components and contaminants: max. 0.05%,-   remainder iron.

This sheet steel was first cold rolled with a thickness reduction of 50%to 96% to an end thickness in the range of about 0.5 mm and thenrecrystallizing-annealed in an induction furnace under inductionheating. For example, an induction coil with 50 kW power at a frequencyof f=200 kHz was used for a sample size of 20×30 [sic]. The annealingcurve is shown in FIG. 1. As can be seen from the annealing curve inFIG. 1, the steel strip was heated within a very short heat-up timet_(A), which typically is between 0.5 s and 10 s, to a maximumtemperature T_(max) above the A₁ temperature (T (A₁)≈725° C.). Themaximum temperature T_(max) lies under the phase transition temperatureT_(f) of the ferromagnetic phase transition (T_(f)≈770° C.). Thetemperature of the steel strip was then maintained at a temperaturevalue above the A₁ temperature for an annealing time t_(G) time of about1 s. During this annealing time t_(G) the steel cooled negligibly fromits maximum temperature T_(max) of, for example, 750° C. to the A₁temperature (about 725° C.). Then the steel strip was cooled to roomtemperature (about 23° C.) by means of a fluid cooling, which can beproduced, for example, by a water cooling or air cooling in a coolinginterval of about 0.25 s. After the cooling, if necessary, another coldrolling step with thickness reduction of up to 40% can take place.

The thus treated sheet steel was then tested with regard to strength andelongation at break. It was shown by comparison experiments that in allcases the elongation at break was higher than 6% and as a rule higherthan 10% and that the tensile strength was at least 500 MPa and in manycases even turned out to be more than 650 MPa.

It was shown by a color etching following Klemm that the sheet steeltreated according to the invention has an alloy structure that hasferrite as the soft phase and martensite and possibly bainite and/orresidual austenite as hard phase. FIG. 2 shows a structure in crosssection with Klemm color etching, where the regions shown there in whiteare the martensite phase and the blue or brown regions indicate theferrite phase. One can see a linear arrangement of the higher strengthphase (martensite/bainite).

It was determined by comparison experiments that the best results withregard to strength-formability are achieved if the heating rate in therecrystallizing induction annealing lies between 200 K/s and 1200 K/sand when the recrystallizing-annealed steel strip is then cooled at acooling rate of more than 100 K/s. Expedient from the standpoint ofequipment here are cooling rates between 350 K/s and 1000 K/s, since inthis case a costly water cooling can be omitted and cooling can takeplace by means of a cooling gas such as air. To be sure, the bestresults with regard to the material properties are achieved when usingwater cooling at cooling rates of more than 1000 K/s.

The sheet steel according to the invention is outstandingly suitable foruse as packaging steel. For example, cans for food or beverages can bemade from the sheet steel according to the invention. Since higherdemands are made on the corrosion resistance of packaging in particularin the food field, it is expedient to provide the sheet steel producedaccording to the invention with a metallic and corrosion resistantcoating after the heat treatment and possibly after a final dressing ora cold rolling step, for example by electrolytic tin plating or chromeplating. However, other coating materials such as aluminum, zinc, orzinc/nickel, and other coating methods, for example hot dip zincplating, can also be used. In each case according to requirements thecoating can take place on one side or both sides.

Compared to the dual-phase steels known from the automobile constructionthe sheet steel according to the invention for use as packaging steel ischaracterized in particular by the considerably lower production costsand by the advantage that a steel with lower alloy concentration andfewer alloy components can be used, so that contamination of thepackaged foods can be avoided. With regard to the strength andformability, the sheet steel according to the invention is comparable tothe dual-phase steels known from automobile construction.

1. Sheet steel for use as packaging steel made from a nonalloy or lowalloy and cold rolled steel having a carbon content of less than 0.1%,wherein the sheet steel contains less than 0.4 wt % manganese, less than0.04 wt % silicon, less than 0.1 wt % aluminum, and less than 0.1 wt %chromium, and has a multiphase structure, which comprises ferrite and atleast one of the structural components martensite, bainite, and/orresidual austenite.
 2. Method for making a packaging steel from a coldrolled sheet steel which is made from a nonalloy or low alloy steelhaving a carbon content of less than 0.1%, wherein the sheet steel isfirst subjected to a recrystallization annealing by means ofelectromagnetic induction at a heating rate of more than 75 K/s and iscooled after the recrystallizing induction annealing at a cooling rateof at least 100 K/s and preferably more than 500 K/s, through which amultiphase structure develops, which comprises ferrite and at least oneof the structural components martensite, bainite, and/or residualaustenite.
 3. Method as in claim 2, wherein the low alloy steel containsless than 0.4 wt % Mn, less than 0.04 wt % Si, less than 0.1 wt % Al,and less than 0.1 wt % Cr.
 4. Sheet steel as in claim 1, wherein themultiphase structure contains more than 80% and preferably at least 95%of the structural components, ferrite, martensite, bainite, and/orresidual austenite.
 5. Sheet steel as in claim 1, wherein the sheetsteel is made from a low alloy steel, which contains boron and/orniobium and/or titanium.
 6. Sheet steel as in claim 1, wherein the sheetsteel is a cold rolled fine or ultrafine sheet.
 7. Sheet steel as inclaim 1, wherein the sheet steel is coated with a surface coating oftin, chromium, aluminum, zinc, or zinc/nickel after therecrystallization annealing and cooling.
 8. Sheet steel as in claim 1,wherein the sheet steel has a tensile strength of at least 500 MPa,preferably more than 650 MPa, and an elongation at break of more than5%, preferably more than 10%, after the recrystallization annealing andcooling.
 9. Sheet steel as in claim 1, wherein the cooling rate at whichthe sheet steel is cooled after the recrystallization annealing isgreater than 100 K/s and preferably greater than 500 K/s.
 10. Sheetsteel as in claim 1, wherein the sheet steel is made from a low alloysteel with the following upper limits for the weight fraction of thealloy components: N: max. 0.02%, Mn: max. 0.4, Si: max. 0.04%, Al: max.0.1%, Cr: max. 0.1%, P: max. 0.03%, Cu: max. 0.1%, Ni: max. 0.1%, Sn:max. 0.04%, Mo: max. 0.04%, V: max. 0.04%, Ti: max. 0.05%, preferablyless than 0.02%; Nb: max. 0.05%, preferably less than 0.02%; B: max.0.005% and other alloying components including contaminants: max. 0.05%.11. Method as in claim 2, wherein the sheet steel after therecrystallizing induction annealing, is cooled by a cooling fluid at acooling rate between 100 K/s and 1000 K/s and preferably at a coolingrate between 350 and 1000 K/s.
 12. Method as in claim 2, wherein therecrystallization annealing takes place in a time interval of 0.5 to 1.5s, preferably about 1 s, where the sheet steel is inductively heated totemperatures above 720° C.
 13. Use of a sheet steel as in claim 1 aspackaging steel, in particular for making cans for foods, beverages, andother materials such as chemical or biological products and for makingaerosol cans and closures.
 14. Method as in claim 2, wherein themultiphase structure contains more than 80% and preferably at least 95%of the structural components, ferrite, martensite, bainite, and/orresidual austenite.
 15. Method as in claim 2, wherein the sheet steel ismade from a low alloy steel, which contains boron and/or niobium and/ortitanium.
 16. Method as in claim 2, wherein the sheet steel is a coldrolled fine or ultrafine sheet.
 17. Method as in claim 2, wherein thesheet steel is coated with a surface coating of tin, chromium, aluminum,zinc, or zinc/nickel after the recrystallization annealing and cooling.18. Method as in claim 2, wherein the sheet steel has a tensile strengthof at least 500 MPa, preferably more than 650 MPa, and an elongation atbreak of more than 5%, preferably more than 10%, after therecrystallization annealing and cooling.
 19. Method as in claim 2,wherein the cooling rate at which the sheet steel is cooled after therecrystallization annealing is greater than 100 K/s and preferablygreater than 500 K/s.
 20. Method as in claim 2, wherein the sheet steelis made from a low alloy steel with the following upper limits for theweight fraction of the alloy components: N: max. 0.02%, Mn: max. 0.4,Si: max. 0.04%, Al: max. 0.1%, Cr: max. 0.1%, P: max. 0.03%, Cu: max.0.1%, Ni: max. 0.1%, Sn: max. 0.04%, Mo: max. 0.04%, V: max. 0.04%, Ti:max. 0.05%, preferably less than 0.02%; Nb: max. 0.05%, preferably lessthan 0.02%; B: max. 0.005% and other alloying components includingcontaminants: max. 0.05%.